IPC-D-279 EN.pdf - 第86页

gas ambient, hydrolyzable dust and sand, solvents and residual hydrolyzable contaminants from the fabrication and assembly processes and handling. Mechanical stresses include sand and dust, mechanical shock, vibration, c…

E-2.0 CHEMICAL

E-2.1 PWA Cleanliness

Where water soluble or other

ﬂuxes are used, assure that the PWA is cleaned at least to

the ionic contamination levels expected of a ﬁrst-pass

board. Otherwise, long term failure mechanisms may

result. Corrosion stress cracking of the solder joints may

occur, together with dendrites on the surface of the printed

board and loss of Surface Insulation Resistance perfor-

mance (particularly important in high impedance linear

applications). See also the comments on cleaning under

‘‘Conformal Coating’’.

E-3.0 MECHANICAL

E-3.1 PWA Flexure

Excessive ﬂexure of the PWA due to

shock, vibration or handling during rework or repair can

result in a cracked component body, a detached termina-

tion, or an overloaded and failed solder joint.

E-3.2 Tooling Impact Impact of a hard tool on a ceramic

component body or termination can result in a cracked or

fractured component body.

During component removal, there should be no mechanical

stress on neighboring components. Some removal tech-

niques and tools have been observed to use a neighboring

component as a pivot or fulcrum.

E-4.0 ELECTRICAL

E-4.1 Electrostatic Discharge (ESD)

Observe ESD pre-

cautions while handling, testing, and transporting the PWA

to avoid the introduction of infant and latent defects.

E-5.0 SMT FAILURES/STRESS CONDITIONS

E-5.1 Component Derating Reference Conditions

Component derating is commonly referenced to the abso-

lute maximum ratings deﬁned by the manufacturer’s speci-

ﬁcation or data sheet. Each of the several maximum ratings

(e.g. power, voltage, current, temperature) must be applied

individually and not in combination with any other abso-

lute maximum rating. Where experience dictates, more

conservative references may be used.

E-5.2 The Most Important Stress and Some Precau-

tions

The most universal stress is temperature. The criti-

cal point at which to evaluate temperature is the active site

or junction or ﬁlm, not the ambient temperature; this deﬁ-

nition of the critical temperature takes into account the

temperature rise in the component due to heat generated

within the component. Many of the common failure

mechanisms in electronic components double their failure

rate contribution with an increase of only 10°C.

Caution The absolute maximum ratings or the

temperature/thermal derating of components usually state

or imply a maximum operating and/or storage temperature

(whether junction or hot-spot) and various electrical values

based upon DC power conditions measured in a still ambi-

ent at 25°C. However, to determine whether this assump-

tion is true for speciﬁc components, you may have to ques-

tion the manufacturers’ engineering staffs.

The physical conﬁguration of the test environment is often

not speciﬁed; this failing most affects low, short-leaded

components such as those in SMT because of unstated sig-

niﬁcant factors such as the orientation of the component,

the printed board conductor conﬁguration, the use (or non-

use) of sockets and the velocity of the airstream immedi-

ately adjacent to the component under test.

E-5.3 Failure Modes/Failure Mechanisms A failure

mode is the failure of a component to perform its electronic

or mechanical function. An electronic component fails due

to the underlying chemical, physical or mechanical mecha-

nisms.

For instance, an integrated circuit (IC) can fail due to an

open failure mode on a given pin or lead.

The responsible failure mechanisms can include: external

lead contamination leading to an open solder joint; lifted

bond wire at the internal package lead; corroded wire in the

package; resistive and brittle intermetallic compound

(IMC) growth at the wire/package interface; broken bond

wire (due to cyclic fatigue from external temperature

cycling, cyclic fatigue from internal power cycling, tensile

overload, material coefficient of thermal expansion incom-

patibles), melted bond wire (due to high currents); lifted

bond wire at the integrated circuit die (due to a poorly

executed bond, corroded bond pat metallization, contami-

nation on bond pad metallization, excessive IMC develop-

ment); open IC die metallization (due to poor oxide step

coverage, electrical overstress, electromigration, corrosion,

stress from dielectric layers, or stress from molding com-

pound movement). Note that these mechanisms can be the

result of simultaneous or sequential exposure of the com-

ponent to the physical/mechanical, chemical or electrical

stresses.

E-6.0 OVERVIEW OF STRESSES

E-6.1 Common Stresses and Component Response to

Stress

Thermal stresses include static high and low ambi-

ent temperature and cyclic ambient temperature; power

cycling results in cyclic internal temperatures. Cyclic tem-

peratures result in thermo-mechanical stresses. Electrical

stresses include static and cyclic power; static, cyclic and

transient voltage; current and current density; electrostatic

discharge (ESD) and electro-magnetic interference (EMI).

Chemical stresses include moisture or humidity, corrosive

IPC-D-279 July 1996

gas ambient, hydrolyzable dust and sand, solvents and

residual hydrolyzable contaminants from the fabrication

and assembly processes and handling. Mechanical stresses

include sand and dust, mechanical shock, vibration,

connection/removal of connectors, and unrelieved strain

due to warped printed circuit boards. Other stresses include

low atmospheric pressure/high altitude/vacuum, electro-

magnetic radiation, ionizing radiation such as X-rays and

alpha particles. A special but not uncommon case of a com-

bined stress is that of temperature- humidity- bias.

E-7.0 IMPORTANCE OF TEMPERATURE AS A COMPO-

NENT STRESS FACTOR

Temperature is the greatest single contributing stress to

component failure.

E-7.1 Temperature-Related Reversible/Temporary

Changes in Component Parameters

Examples of revers-

ible changes include:

• increased resistance in metallic resistors

• changes in capacitance, dielectric constant, power fac-

tor, and equivalent series resistance (ESR) of capaci-

tors

• decreased inductance of inductors and ﬁlters, particu-

larly near Tc (Curie temperature)

• increased gain of bipolar transistors

• decreased breakdown voltage and increased saturation

(leakage) current of P-N junctions

• increased output resistance of bipolar and ﬁeld effect

transistors

• decreased viscosity and load carrying ability of lubri-

cants.

High temperature exposure during SM assembly process-

ing (particularly solder reﬂow) or service also results in

reversible

• expansion of materials (particularly of polymers above

) with the possibility of the jamming of moving

parts and the ‘‘bi-metallic’’ bowing of materials

• loss of control and a long recovery period in tempera-

ture stabilized components such as crystal oscillators.

E-7.2 Temperature-Related Irreversible/Permanent

Changes in Component Parameters

Examples of irre-

versible effects of exposure to high temperatures include

the following:

• oxidation (particularly of lubricants and contacts)

• corrosion (particularly in the presence of moisture and

hydrolyzable contaminants)

• Intermetallic compound (IMC) formation particularly

in the case of SMT and Tape Automated Bonded sol-

der joints and other bi-metallic joints.

• grain growth in multiphase alloys such as eutectic

lead-tin solder

• diffusion of alkali metals in semiconductor devices

resulting in device instability

• diffusion of halogenated solvents through rubber seals

of non-solid electrolytic capacitors resulting in internal

corrosion and subsequent component failure

• evaporation and subsequent loss of high vapor pres-

sure fractions of silicone compounds, greases and ﬂu-

ids

• evaporation and subsequent loss of plasticizers in plas-

tics

• evaporation and transportation of silicone and plasti-

cizer compounds to the mating surfaces of separable

contacts such as those in relays and connectors and

cold ﬂow of materials such as polytetraﬂuoroethylene

(PTFE).

• evaporation and subsequent loss of ﬂuid in non-solid

electrolytic capacitors has caused losses on compo-

nents and instruments which have been in storage at

room temperatures. Evaporation is accelerated at

higher temperatures and higher altitudes. Some alumi-

num electrolytic capacitors rated for operation from

−55°C to +85°C or +105°C contain ﬂuids such as dim-

ethyl formamide (DMF) which has a boiling point of

67°C. Replacement of the DMF with dimethyl aceta-

mide (BP = 74°C) or gamma butyrolactone (GBL) (BP

= 97°C) reduces the susceptibility of the component to

high temperatures. Because these solvents have ﬂash-

points very close to their boiling point, the solvent

change also reduces the ﬁre hazard. Changing from

DMF to dimethyl acetamide or GBL also reduces the

toxicity of any leaking electrolyte.

High temperature exposure during SM assembly process-

ing (particularly solder reﬂow) also results in irreversible

• internal and external SM integrated circuit package

delamination and cracking, mechanical damage to the

surface of the die and its microinterconnections as well

as and potential failure due to corrosion of the die

metallization. Passive component networks are also

susceptible to this delamination, cracking, and corro-

sion. See appendix D for a summary and IPC-SM-786

for details.

• temporary softening and a degradation in resistance to

cut-through of insulating polymers (such as those in

capacitors) with a permanent loss of dielectric with-

stand strength and mechanical strength and a long

period to recover some of the lost properties.

• stress cracking of susceptible polymers, such as trans-

parent nylon optical components under internal

July 1996 IPC-D-279

(molded-in) or external mechanical stress and particu-

larly in the presence of some solvents (such as con-

densing alcohol vapor)

• mechanical stressing of components containing mate-

rials with mismatched coefficients of thermal expan-

sion (CTE) such as joint stress between a polymer-

glass based SM substrate and rigid ceramic

components such as large multilayer capacitors, power

resistors, ceramic based hybrids, and inductors

• melting and opening of soldered connections internal

to capacitors, inductors, crystals, and resistor/capacitor

networks

• melting or softening of the polymeric capacitor dielec-

trics such as polystyrene, polycarbonate (PC), polypro-

pylene, polyethylene terepthalate (PET) with dielectric

thinning as a result of the relaxation of winding

stresses at high temperature. The thinned dielectric

results in uncontrolled increase in capacitance and

decrease in dielectric breakdown voltage and a long

period to recover some of the lost properties

• expansion of elastomeric materials such as silicone or

RTV used for ‘‘potting’’ components (such as optocou-

plers, pulse transformers, inductors, and delay lines)

with subsequent breaking of components, wires and

joints

• softening and relaxation of elastomeric materials such

as O-rings in variable resistors, with subsequent loss of

seal and initiation of corrosion

• softening and cracking of the low T

conformal coat-

ings of axial and radial capacitors, with subsequent

corrosion

• softening and weakening of internal epoxy connections

in assemblies such as crystals and hybrid oscillators

• softening, distortion, or deformation of plastic compo-

nents (such as surface mount connectors and light

emitting diode [LED] display scramblers) with loss of

dimensional accuracy

• overcuring of polymers used for insulation with a

decrease in insulation resistance (IR)

• boiling and evaporation of the ﬂuids in non-solid elec-

trolytic (such as aluminum and wet slug tantalum)

capacitors with subsequent loss of capacitance and

increased Equivalent Series Resistance.

Identify the maximum allowable internal body temperature

(deﬁned by the boiling point of the electrolyte, the soften-

ing point of the plastic dielectric), or the softening range of

the internal solder joints and match that temperature con-

straint with the solder reﬂow proﬁle to prevent component

degradation.

See also the comments in this design guide, Appendix A,

on the effects of temperature and temperature cycling on

solder joint reliability and solder joint fatigue and methods

of computing the reliability impact. See also the comments

on process and rework temperatures in this design guide,

particularly the effects on plastic surface mount compo-

nents in Appendix F.

E-7.3 Effects of Low Temperature Low temperatures

result in:

• loss of ﬂexibility and decreased impact resistance in

polymers

• liquid water ﬁlm formation below dewpoint with sub-

sequent opportunity to induce corrosion

• ice formation with subsequent delamination or melting

of the ice

• viscosity increase particularly in lubricants and liquid

electrolytes

• contraction of materials with subsequent jamming of

moving parts or bi-metallic bowing of materials

• thermomechanical stress of components containing

materials with mismatched coefficient of thermal

expansion (∆) joined by SM reﬂow

• decreased bipolar transistor gain and increase FET

transconductance

• stress and rupture of some SM solder masks and other

coatings

• loss of control in temperature stabilized components

such as crystal oscillators

• increased dissipation factor in ‘‘hi-k’’ ceramic capaci-

tors

• increased stress in encapsulants and molding com-

pounds with subsequent damaged IC passivation, dam-

aged IC metallization, cracked silicon, or induced dark

line defects and loss of light output in LEDs.

E-7.4 Effects of Temperature Changes Temperature or

thermal cycling can result in:

• repeated stressing of structures and material systems

with mismatched thermal expansions resulting in

repeated ‘‘bi-metallic’’ bowing and possible fatigue.

Particularly susceptible are systems of ceramic and

organic components affixed to organic and ceramic

substrates, respectively (for instance, where ceramic

LCC are affixed to organic FR-4 boards or plastic

encapsulated components are affixed to ceramic sub-

strates). The most severe cases can result in compo-

nent cracking or solder joint failure due to overload.

Thermo-mechanical fatigue effects are worse with

lower cycling frequencies due to relaxation effects.

• jamming of moving parts

• repeated condensation of moisture

• repeated evaporation of moisture

IPC-D-279 July 1996